The present invention relates to light emitting diodes. Light emitting diodes (LEDs) are a class of photonic semiconductor devices that convert an applied voltage into light by encouraging electron-hole recombination events in an appropriate semiconductor material. In turn, some or all of the energy released in the recombination event produces a photon. When recombination events produce photons, they initiate photons in all directions.
Light emitting diodes share a number of the favorable characteristics of other semiconductor solid-state devices. These include generally robust physical characteristics, long lifetime, high reliability, and, depending upon the particular materials, low cost. These physical characteristics, along with relatively low power requirements, make LEDs desirable as light output devices. The general theory and operation of LEDs are well understood in the art. Appropriate references about the structure and operation of light emitting diodes include S. M. SZE, PHYSICS OF SEMICONDUCTOR DEVICES (2d ed. 1981) and E. FRED SCHUBERT, LIGHT-EMITTING DIODES (2003).
Gallium Nitride (GaN)-based LEDs typically comprise an insulating or semiconducting substrate such as silicon carbide (SiC) or sapphire on which a plurality of GaN-based epitaxial layers are deposited. These epitaxial layers form an active region having a p-n junction, which emits light when energized. LEDs can be mounted substrate side down onto a submount or onto a lead frame, or both. Alternatively, flip-chip LEDs are mounted on the lead frame with the substrate side facing up; i.e., with the substrate side facing away from the submount.
Flip-chip mounted LEDs often incorporate vertical geometry. The term “vertical” does not necessarily describe the final position of the overall device, but instead describes an orientation within the device in which the electrical contacts used to direct current through the device and its p-n junction are positioned on opposite faces (axially) from one another in the device. Thus, in a typical form, a vertical device includes a conductive substrate, a metal contact on one face of the substrate, two or more epitaxial layers on the opposite face of the substrate to form the p-n light-emitting junction, and a top contact on the top epitaxial layer to provide a current path through the layers and their junction and through the substrate to the substrate contact.
Many LEDs employ reflective layers to improve their external light output. Such Reflective layers are typically positioned to redirect photons toward the desired emitting surface and away from any photon absorbing material. Reflective layers are typically made of metals such as Ag or Al. Metallic reflective layers have the advantage of reflecting light at arbitrary angles and polarizations, but have a higher optical loss than other reflectors. Dielectric mirrors, such as distributed Bragg reflectors (DBRs), have also been used as reflectors. DBRs have low optical loss and high reflectance, but have the disadvantage of effectively reflecting only that light which impinges near the direction normal to the DBR. A superior dielectric mirror has been demonstrated by using an omnidirectional reflector (ODR) composed of alternating layers of TiO2 and SiO2. C. H. Lim et al., Enhancement of INGaN-GaN Indium-Tin-Oxide Flip-Chip Light-Emitting Diodes With TiO2—SiO2 Multilayer Stack Omnidirectional Reflector, 18 IEEE PHOTONICS TECHNOLOGY LETTERS 2050 (2006); see also H. W. Huang et al., High Performance GaN-Based Vertical-Injection Light-Emitting Diodes With TiO2—SiO2 Omnidirectional Reflector and n-GaN Roughness, 19 IEEE PHOTONICS TECHNOLOGY LETTERS 565 (2007). An ODR has superior reflectance when compared to a DBR, because the ODR reflects light at any incidence angle and polarization.
When a metallic reflective layer is incorporated into a vertical flip chip LED, the metallic reflective layer can also include additional metal layers to serve to connect the p-layer to the conductive substrate. Making such an electrical connection typically requires high temperature processing. When a dielectric reflective layer, such as a DBR, is incorporated into a vertical flip chip LED, some other means must be made to connect the p-layer to the conductive substrate because dielectric reflective layers are electrically insulating. Present devices form this connection by creating via holes in the dielectric layers and filling the holes with one or more metals, which serve as conducting contacts. Although such vias provide the desired current path, they tend to absorb light and reduce the reflective advantages of the surrounding dielectric reflective layer. Thus, such vias can limit the benefits of using dielectric reflectors and can reduce the overall external quantum efficiency of LEDs that incorporate such mirrors and vias.
Accordingly, a need exists for LEDs with dielectric reflective layers that avoid or minimize the use of such light absorbing structures.